Re: [PATCH -next v3] cpuset: Treat cpusets in attaching as populated

[Waiman Long <llong@redhat.com>]

On 11/13/25 9:08 PM, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>
>
> Currently, the check for whether a partition is populated does not
> account for tasks in the cpuset of attaching. This is a corner case
> that can leave a task stuck in a partition with no effective CPUs.
>
> The race condition occurs as follows:
>
> cpu0				cpu1
> 				//cpuset A  with cpu N
> migrate task p to A
> cpuset_can_attach
> // with effective cpus
> // check ok
>
> // cpuset_mutex is not held	// clear cpuset.cpus.exclusive
> 				// making effective cpus empty
> 				update_exclusive_cpumask
> 				// tasks_nocpu_error check ok
> 				// empty effective cpus, partition valid
> cpuset_attach
> ...
> // task p stays in A, with non-effective cpus.
>
> To fix this issue, this patch introduces cs_is_populated, which considers
> tasks in the attaching cpuset. This new helper is used in validate_change
> and partition_is_populated.
>
> Fixes: e2d59900d936 ("cgroup/cpuset: Allow no-task partition to have empty cpuset.cpus.effective")
> Signed-off-by: Chen Ridong <chenridong@huawei.com>
> ---
>   kernel/cgroup/cpuset.c | 35 +++++++++++++++++++++++++++--------
>   1 file changed, 27 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index daf813386260..8bf7c38ba320 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -356,6 +356,15 @@ static inline bool is_in_v2_mode(void)
>   	      (cpuset_cgrp_subsys.root->flags & CGRP_ROOT_CPUSET_V2_MODE);
>   }
>   
> +static inline bool cpuset_is_populated(struct cpuset *cs)
> +{
> +	lockdep_assert_held(&cpuset_mutex);
> +
> +	/* Cpusets in the process of attaching should be considered as populated */
> +	return cgroup_is_populated(cs->css.cgroup) ||
> +		cs->attach_in_progress;
> +}
> +
>   /**
>    * partition_is_populated - check if partition has tasks
>    * @cs: partition root to be checked
> @@ -373,19 +382,29 @@ static inline bool is_in_v2_mode(void)
>   static inline bool partition_is_populated(struct cpuset *cs,
>   					  struct cpuset *excluded_child)
>   {
> -	struct cgroup_subsys_state *css;
> -	struct cpuset *child;
> +	struct cpuset *cp;
> +	struct cgroup_subsys_state *pos_css;
>   
> -	if (cs->css.cgroup->nr_populated_csets)
> +	/*
> +	 * We cannot call cs_is_populated(cs) directly, as
> +	 * nr_populated_domain_children may include populated
> +	 * csets from descendants that are partitions.
> +	 */
> +	if (cs->css.cgroup->nr_populated_csets ||
> +	    cs->attach_in_progress)
>   		return true;
>   
>   	rcu_read_lock();
> -	cpuset_for_each_child(child, css, cs) {
> -		if (child == excluded_child)
> +	cpuset_for_each_descendant_pre(cp, pos_css, cs) {
> +		if (cp == cs || cp == excluded_child)
>   			continue;
> -		if (is_partition_valid(child))
> +
> +		if (is_partition_valid(cp)) {
> +			pos_css = css_rightmost_descendant(pos_css);
>   			continue;
> -		if (cgroup_is_populated(child->css.cgroup)) {
> +		}
> +
> +		if (cpuset_is_populated(cp)) {
>   			rcu_read_unlock();
>   			return true;
>   		}
> @@ -670,7 +689,7 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
>   	 * be changed to have empty cpus_allowed or mems_allowed.
>   	 */
>   	ret = -ENOSPC;
> -	if ((cgroup_is_populated(cur->css.cgroup) || cur->attach_in_progress)) {
> +	if (cpuset_is_populated(cur)) {
>   		if (!cpumask_empty(cur->cpus_allowed) &&
>   		    cpumask_empty(trial->cpus_allowed))
>   			goto out;
Reviewed-by: Waiman Long <longman@redhat.com>